Recently, people who travel in automobiles are paying attention to electric power-assisted bicycles for their health and the environment. Among the electric power-assisted bicycles, a bicycle capable of energy regeneration and long distance travel with a single charging came into focus. Generally, a regenerative charging is performed after a braking is applied. However, energy efficiency is low when regenerative charging is performed after the braking is applied. Therefore, it is preferable that the regenerative charging starts once a brake lever is pulled before a mechanical brake is actually applied. In order to achieve this, means capable of detecting the pulling of the brake lever before the mechanical brake is actually applied, i.e., a tension of a brake wire, and capable of measuring a small amount of movement (displacement) proportional to the tension of the brake wire is necessary.
FIGS. 16A and 16B show relationships between an amount of manipulation of a brake lever and a braking force in an electric power-assisted vehicle. For the electric power-assisted bicycle described above, it is necessary to measure the amount of manipulation of the brake lever corresponding to an amount the movement of the brake wire in an idle period shown in FIG. 16A when the pulling of the brake lever starts. Next, it is necessary to detect a point (an operation point P1 of the mechanical braking) where a brake pad starts the mechanical braking of the bicycle by inhibiting the rotation of wheels based on the extension of the brake wire. This is due to the fact that rider(s) may feel as if the bicycle underwent an abrupt braking or the braking force were insufficient when a braking control between the regenerative braking and the mechanical braking is not performed smoothly before and after the braking.
In particular, an idle period of a brake in an electric power-assisted bicycle or the like can change by replacing braking wires or by adjusting the tension of the brake wire. Accordingly, as shown in FIG. 16B, the amount of manipulation of the brake lever required for starting the mechanical braking may easily deviate from the operation point P1 to another operation point P2 of the mechanical brake. According to the prior art, only the amount of manipulation of the brake lever is detected, and start of the mechanical braking is determined when the detected amount of manipulation reaches a predetermined amount. Therefore, according to the prior art, since the start of mechanical braking cannot be accurately detected even when the operation point is changed to P2 as described above, the braking control cannot be performed smoothly between the regenerative braking and the mechanical braking. In order to maximize the efficiency of the regenerative charging, a configuration is necessary wherein the start of mechanical braking is precisely detected by simultaneously or sequentially measuring both the amount of movement and an amount of the extension of the brake wire.
Conventionally, an optical interferometer is used to measure a minute displacement such as the amount of the movement or the amount of the extension of the brake wire. A Michelson interferometer 200 shown in FIG. 17A includes a laser light source 202; a collimating lens 204 configured to convert an incident beam of laser light into a parallel beam; a splitter 206 configured to divide a beam into two beams and direct one beam to a fixed minor 208 and the other beam to a movable mirror 210; and an optical sensor 212 configured to receive an interference beam of the two reflected beams. In the Michelson interferometer 200, a detector detects two optical patterns of bright and dark bands when the movable mirror 210 moves one wavelength with respect to a fixed unit 214 in a direction of beam propagation. As shown in FIG. 17B, these optical interference fringes are observed as interference patterns 216. In this case, a displacement not greater than the one wavelength may be detected by detecting a voltage gradient of the interference fringe. In addition, a displacement greater than the one wavelength may be measured by counting the number of the interference fringes (i.e., the interference patterns). As shown in FIG. 17C, the displacement may be calculated by equation: [displacement]=[the one wavelength]×[the number of interference fringes]×2 because path difference in round trip is twice the displacement of the movable mirror. Here, additional means are necessary to detect the direction of the movement of the brake wire. Techniques using optical interference such as a device and a method of detecting a phase difference described above are disclosed in the following Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-271624.